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Firearms & Tool Marks Forensic Science.

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Presentation on theme: "Firearms & Tool Marks Forensic Science."— Presentation transcript:

1 Firearms & Tool Marks Forensic Science

2 Copyright and Terms of Service
Copyright © Texas Education Agency, These materials are copyrighted © and trademarked ™ as the property of the Texas Education Agency (TEA) and may not be reproduced without the express written permission of TEA, except under the following conditions: 1)  Texas public school districts, charter schools, and Education Service Centers may reproduce and use copies of the Materials and Related Materials for the districts’ and schools’ educational use without obtaining permission from TEA. 2)  Residents of the state of Texas may reproduce and use copies of the Materials and Related Materials for individual personal use only, without obtaining written permission of TEA. 3)  Any portion reproduced must be reproduced in its entirety and remain unedited, unaltered and unchanged in any way. 4)  No monetary charge can be made for the reproduced materials or any document containing them; however, a reasonable charge to cover only the cost of reproduction and distribution may be charged. Private entities or persons located in Texas that are not Texas public school districts, Texas Education Service Centers, or Texas charter schools or any entity, whether public or private, educational or non-educational, located outside the state of Texas MUST obtain written approval from TEA and will be required to enter into a license agreement that may involve the payment of a licensing fee or a royalty. Contact TEA Copyrights with any questions you may have.

3 Type of Firearms Handguns Pistol – a handheld firearm
Revolver – a pistol with a revolving cylinder (features several firing chambers within the cylinder) Semiautomatic – a pistol with a clip-fed mechanism that fires one shot per pull of the trigger; the empty cartridge ejects and the next cartridge advances automatically (features a removable magazine) Fully-automatic – a firearm with a clip-fed mechanism that fires repeatedly and for as long as the trigger is held down

4 Type of Firearms (continued)
Long Guns may be single-shot, repeating, semi-automatic, or automatic Examples Rifle – a firearm that has a long barrel; a long gun

5 Type of Firearms (continued)
Shotgun – uses shell ammunition that contains numerous ball- shaped projectiles, called shot The narrowing of the smooth barrel, called the choke of the shotgun, can concentrate the shot when fired Gauge is the diameter of the shotgun barrel; originally the number of lead balls having the same diameter as the barrel that would weigh a pound The higher the gauge number, the smaller the barrel’s diameter The only exception is the .410 shotgun, in which bore size is inches

6 Discharging the Firearm
Pulling the trigger releases the weapon’s firing pin, causing it to strike the primer – the primer ignites the powder The expanding gases generated by the burning gunpowder propel the bullet forward through the barrel The shell is impressed with markings by its contact with the metal surfaces of the weapon’s firing and loading mechanisms Impressions found on the weapon and markings found on the bullet are used by forensic scientists for examination and comparison

7 Firearm Analysis Gun Barrel Markings
The barrel’s inner surface leaves its markings on any bullet that passes through it (these markings are unique to each gun) The gun barrel is produced from a solid bar of steel that has been hollowed out by drilling The microscopic drill marks left on the barrel’s inner surface are randomly irregular and make each barrel unique

8 Firearm Analysis (continued)
Gun Barrel Markings (continued) Barrels are also manufactured with spiral grooves, known as rifling The parts of the original bore left between the grooves are called lands The grooves guide a fired bullet through the barrel, giving it a rapid spin to insure accuracy The grooves guide the bullet through the barrel, giving it a rapid spin to ensure accuracy

9 Firearm Analysis (continued)
Gun Barrel Markings (continued) The diameter of the gun barrel, measured between opposite lands, is known as the caliber (expressed in hundredths of an inch or millimeters – for example, .22 caliber or 9 millimeter) Once a manufacturer chooses a rifling process, the class characteristics of the weapon’s barrel will remain consistent Each will have the same number of lands and grooves, with the same approximate width and direction of twist

10 Firearm Analysis (continued)
Striations Fine lines in the interior of the barrel Impressed into the metal as the negatives of minute imperfections found on the rifling cutter’s surface Or produced by minute chips of steel pushed against the barrel’s inner surface by a moving broach cutter Striations form the individual characteristics of the barrel The inner surface of the barrel leaves its striation markings on any bullet passing through it

11 Firearm Analysis (continued)
Bullets No two rifled barrels, even those manufactured in succession, will have identical striation markings The number of lands and grooves, and their direction of twist, are easy points of comparison during the initial stages of an examination between an evidence bullet and a test-fired bullet Any differences in these class characteristics immediately eliminate the possibility that both bullets traveled through the same barrel

12 Firearm Analysis (continued)
Cartridge Comparison The firing pin, breechblock, and ejector and extractor mechanism also create distinctive signatures on cartridge cases The shape of the firing pin will be impressed into the relatively soft metal of the primer on the cartridge case The cartridge case, travelling rearward, is impressed with the surface markings of the breechblock

13 Firearm Analysis (continued)
Cartridge Comparison (continued) Other distinctive markings that may appear on the shell as a result of metal-to-metal contact are caused by the Ejector –the mechanism on a firearm that throws the cartridge or fired case from the firearm Extractor – the mechanism by which the cartridge of a fired case is withdrawn from the firing chamber Magazine or clip –the mechanism that holds the bullets

14 Identification Comparison Microscope
The single most important tool for a firearms examiner Two bullets can be observed and compared simultaneously within the same field of view Not only must the lands and grooves of the test and evidence bullet have identical widths, but the longitudinal striations on each must be the same

15 Identification (continued)
Computerized Imaging Computerized imaging technology has made it possible to store bullet and cartridge surface characteristics similarly to automated fingerprint files The National Integrated Ballistics Information Network (NIBIN) produces database files from bullets and cartridge casings retrieved from crime scenes or test fires from retrieved firearms, often linking a specific weapon to multiple crimes It is important to remember, however, that the ultimate decision for making a final comparison must be determined by the forensic examiner through traditional microscopic methods

16 Residue Firearm Residue
Gunshot residue (GSR) – tiny particles expelled from a firearm when it is fired When a firearm is discharged, the bullet, unburned and partially burned particles of gunpowder, and smoke are propelled out the barrel toward the target If the muzzle of the weapon is sufficiently close, these products will be deposited onto the target The distribution of gunpowder particles and other residues around a bullet hole allows for an estimation of the distance from which a handgun or rifle was fired

17 Residue (continued) Firearm Residue (continued)
The precise distance from which a handgun or rifle has been fired is determined through careful comparison of the powder-residue pattern located on the victim’s clothing or skin against test patterns the suspect weapon males when fired at varying distances from a target By comparing the test and evidence patterns, the examiner may find enough similarity in shape and density to formulate an opinion about the distance from which the shot was fired

18 Residue (continued) Firearm Residue (continued)
When the weapon is held in contact with (or less than 1 inch from) the target, it creates a star-shaped (stellate) tear pattern around the bullet hole entrance, rimmed by a smokeless deposit of vaporous lead A halo of vaporous lead (smoke) deposited around a bullet hole normally indicates a discharge of 12 to 18 inches or less Scattered specks of unburned and partially burned powder grains without any accompanying soot are often observed at distances up to 25 inches (and occasionally as far as 36 inches)

19 Residue (continued) Firearm Residue (continued)
More than 3 feet will not usually result in deposits of powder residue, and the only visual indication will be a dark ring around the hole, known as a bullet wipe When garments or other evidence relevant to a shooting are taken to the crime laboratory, the surfaces of all items are first examined microscopically for the presence of gunpowder residue

20 Residue (continued) Firearm Residue (continued)
Chemical tests, such as the Greiss test, may be needed to detect gunpowder residues that are not visible The firing distances for shotguns must again be determined through test firing. The muzzle to target distances can be established by measuring the spread of the discharged shot

21 Residue (continued) Primer Residue on Hands
Firing a weapon also propels residues back toward the shooter Traces of these residues are often deposited on the shooter’s firing hand, and detection can provide valuable information as to whether an individual has recently fired a weapon

22 Residue (continued) Primer Residue on Hands (continued)
Examiners measure the amount of barium and antimony on the suspect’s hands, particularly the thumb web, the back of the hand, and the palm They may also characterize the morphology  of particles containing these elements to determine whether a person has fired, handled, or was near a discharged firearm

23 Trajectory Is the flight path of a projectile
Can be calculated by finding two reference points along the flight path of the projectile The reference points can be a bullet hole in an object, such as a wall or a window, or a bullet wound on a victim

24 Trajectory (continued)
Investigators Look for clues at a crime scene to help calculate a bullet’s trajectory and figure out where a shooter discharged the firearm Might position the corpse (in cases involving a victim’s body) as it was at the time of impact and use a metal or wooden dowel to indicate the path of the bullet Can also use lasers to trace straight paths to determine the position of the shooter or shooters

25 Trajectory (continued)
Gravity and Trajectory Two major forces are acting on a bullet once it is fired: the forward force of the gunshot and the downward force of gravity A bullet begins to drop as it leaves the barrel of a firearm If the shot is taken at a very distant object, the line of sight of the target must be adjusted to compensate for the effect of gravity on the bullet If the target is closer, there would be less adjustment Wind speed and direction are also factors affecting adjustments the shooter must make to hit the target

26 Trajectory (continued)
Locating the Shooter (Example) A bullet is shot and found in a vehicle The bullet first penetrated the front driver’s side window and then the seat The bullet may have come from a building across the street Police need to recreate the crime scene and determine the path of the bullet using the hole in the car’s window and the bullet hole in the seat as their reference points Using a laser beam, they project a line creating the approximate trajectory path of the bullet from the building to the car

27 Trajectory (continued)
Locating the Shooter (Example) (continued) Investigators must also measure the distance from the car to the building To determine the position of the shooter, they must determine the distance between the shooter and the bullet hole in the car seat This requires at least two reference points from which to project a line back to the source (the shooter in the building) To determine the distance between the shooter and the hole in the car seat, investigators must set up a direct proportion using the two right angles

28 Trajectory (continued)
Calculation (Distance vs. Drop) Measure how many feet the bullet hole is above the ground Attempt to locate the bullet’s origin, and measure the distance from the two reference points Measure the horizontal distance from the broken window to the bullet hole horizontally and compare this distance to the diagonal length of the bullet path from the hole in the car’s window to the bullet hole

29 Trajectory (continued)
Calculation (Distance vs. Drop) (continued) Distance to the window = Distance to the shooter (c) Distance along horizon Distance to the side of the building c = _____ c (the hypotenuse) = the distance to the shooter a = distance to the building b = the height of the shooter from the horizon (not from the ground) c2 = a2 + b2 b = ____in. ~ ____ft.

30 Trajectory (continued)
Calculation (Distance vs. Drop) (continued) Compare the distance from the building with the height of the bullet hole (determined in step # 6) and the horizon With this information the investigator can determine where the shooter was, and at what height (or floor) the bullet originated

31 Firearm Evidence Collection
Firearms are collected by holding the weapon by the edge of the trigger guard or the checkered portions of the grip Before the weapon is sent to the laboratory, all precautions must be taken to prevent accidental discharge of a loaded weapon (in most cases, it will be necessary to unload the weapon) When a revolver is recovered, the chambers, their positions, and corresponding cartridges must be recorded Firearm evidence must be marked for identification (usually a tag on the trigger guard), and a chain of custody must be established

32 Firearm Evidence Collection (continued)
Bullets recovered at the crime scene are scribed with the investigator’s initials, either on the base or the nose of the bullet The obliteration of striation markings that may be present on the bullet must be scrupulously avoided The investigator must protect the bullet by wrapping it in tissue paper before placing it in a pillbox or an envelope for shipment to the crime laboratory Fired casings must be identified with the investigator’s initials placed near the outside or inside mouth of the shell Discharged shotgun shells are initialed on the paper or plastic tube or on the metal nearest the mouth of the shell

33 Tool Marks and Other Impressions
A tool mark is any impression, cut, gouge, or abrasion caused by a tool coming into contact with another object A careful examination of the impression can reveal important class characteristics, such as the size and shape of the tool It is the presence of minute imperfections on a tool that imparts individuality to it The shape and pattern of such imperfections are further modified by damage and wear during the life of the tool

34 Tool Marks and Other Impressions (continued)
Tool Marks (continued) The comparison microscope is used to compare crime scene tool marks with test impressions made with the suspect tool When practical, the entire object or the part of the object bearing the tool mark should be submitted to the crime laboratory for examination Under no circumstances should the crime scene investigator attempt to fit the suspect tool into the tool mark Any contact between the tool and the marked surface may alter the mark and will, at the least, raise serious questions about the integrity of the evidence

35 Tool Marks and Other Impressions (continued)
Abrasion mark – a mark produced when a surface slides across another surface Cutting mark – a mark produced along the edge as a surface is cut Indentation mark – a mark or impression made by a tool on a softer surface Impressions of other kinds, such as shoe, tire, or fabric, may be important evidence Before any impression is moved or otherwise handled, it must be photographed (including scale) to show all the observable details of the impression

36 Tool Marks and Other Impressions (continued)
If the impression is on a readily recoverable item, such as glass, paper, or floor tile, the evidence is transported intact to the laboratory If the surface cannot be submitted to the laboratory, the investigator may be able to preserve the impression in a manner similar to lifting a fingerprint

37 Tool Marks and Other Impressions (continued)
When shoe and tire marks are impressed into soft earth at a crime scene, their preservation is best accomplished by photography and casting In areas where a bloody footwear impression is very faint or where the subject has tracked through blood, leaving a trail of bloody impressions, chemical enhancement can visualize latent or nearly invisible blood impressions

38 Points of Comparison A sufficient number of points of comparison, or the uniqueness of such points, will support a finding that both the questioned and test impressions originated from the same source New computer software and websites may be able to assist in making shoeprint and tire impression comparisons Also, bite mark impressions on skin and foodstuffs have proven to be important evidence in a number of homicide and rape cases

39 Resources Texas Education Agency, Forensic Certification Training, Sam Houston State University Forensic Science: Fundamentals & Investigation (1st Edition), Anthony Bertino Forensic Science: From the Crime Scene to the Crime Lab (1st Edition), Richard Saferstein The Science Spot – Forensic Science Investigator/Officer’s Personal Experience To find the video lectures do an Internet search for the following: CSI and “Impressions” Richard Saferstein CSI and Firearms Richard Saferstein

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